Content-Type Header Handling

[SProst]

If Content-Type does not begin with "application/grpc", gRPC servers SHOULD respond with HTTP status of 415 (Unsupported Media Type). This will prevent other HTTP/2 clients from interpreting a gRPC error response, which uses status 200 (OK), as successful.

In the headers / metadata parsing function receiveMetadata in the server-call class it might be a good idea to check this header and issue a 415 response if the header doesn't comply per the protocol.

[cjihrig]

Thanks for finding these missing edge cases.